In present systems, when a tractor connects to a trailer, the operator completes an electrical connection through a cable between the two units to provide electrical power to the trailer for operation of the lights and the anti-lock braking system. Various devices serve to indicate when the connection is broken. However, when the tractor's ignition is switched off while still connected to a trailer, such as during a stop en route for the driver to sleep or eat the trailer has no way of determining whether the tractor has in fact been decoupled or if the tractor has merely been shut off.
An object of this invention is to overcome this and other problems in the system.